The present invention relates to a novel method for the production of an assembly for a nuclear reactor, as well as to an assembly obtained by this method.
The fertile or fuel assemblies used in fast neutron nuclear reactors essentially comprise three parts. Thus, starting from the top, each assembly comprises an upper neutron protection, an intermediate (fissile or fertile) part and a foot or base.
More specifically, the foot or base of the assembly comprises a cylindrical tube welded by its upper end to a solid annular bearing member and by its lower end to a plug having a lock. The annular bearing member has a spherical bearing surface by which the assembly rests on a support which also serves to supply liquid sodium to the assembly. For this purpose, the tube has openings and the bearing member is traversed by an axial passage. The intermediate part comprises a bundle of fuel rods fixed at their upper end to an attachment grid and located in a tube having a polygonal and generally a hexagonal cross-section. Finally, the upper neutron protection is constituted by a solid member, whose centre is traversed by a passage by which the reheated liquid sodium leaves the assembly.
At present, the connection between the three parts of the assembly is brought about by welding the ends of the hexagonal tube to the upper neutron protection and to the bearing member of the assembly foot.
Bearing in mind this latter feature, at least one of the welds connecting the hexagonal tube to the solid parts constituted by the upper neutron protection and the bearing member of the assembly foot must be produced after fitting the bundle of rods into the tube. At present, the bundle of rods is fitted before the production of these two welds.
This production procedure is not satisfactory, particularly for the following reasons.
The making of the welds when the bundle of fuel rods is in place must take place behind a neutron protection. Thus, the welding, inspections and possible repair operations have to be carried out remotely, which involves long and costly handling operations with respect to the assembly.
In view of the safety requirements, a weld having even a very small crack (e.g. of length exceeding 0.2 mm) must be considered as unsatisfactory. However, the presence of the fuel rod bundle within the hexagonal tube makes the inspection of the welds much more difficult. Thus, the more accurate direct, non-destructive inspection methods cannot be used in these conditions and other methods, such as ultrasonic testing, does not make it possible to detect such small cracks.
It is therefore conventional practice to use an inspection procedure consisting of taking testpieces from sample tubes during the production of the assemblies, followed by the performance of appropriate tests and inspections on these testpieces. Thus, such an inspection takes place with a time lag and the results thereof are only available after carrying out one or two series of welds (each series consisting e.g. of 20 welds). Therefore, when a doubtful testpiece has been detected, it is necessary to scrap, or at least consider as dubious, a large number of finished assemblies. Therefore, this method suffers from very high costs and can lead to the immobilization of a large amount of fissile material.
The disadvantage referred to hereinbefore is not only theoretical, because the different geometry of the parts to be welded (relatively thin hexagonal tube and thick solid parts) leads to the formation of an asymmetrical weld bead and causes differential expansions and consequently thermal stresses, which in each case lead to a risk of cracking during welding.